>> Agreed.  Simplicity is the hallmark of Emacs.
>
> Wouldn't this
>
> -  (let* ((files (dired-get-marked-files t arg))
> +  (let* ((files (or (dired-get-marked-files t arg)
> +                    (user-error "No files specified")))
>
> call for an extra argument to 'dired-get-marked-files' to emit the
> user error right there?  If it's TRT in your cases, it might give
> coders a heads-up that it's TRT in their cases as well.

Usually we have an arg NOERROR to not signal an error when it's non-nil.
But since we should keep backward-compatibility, the arg should be
opt-in rather than opt-out here: